1. Field of the Invention
This invention relates to the field of tubular connections and methods for connecting tubular members, particularly for oilfield connections of tapered threaded pin and box members.
2. Background Description
Pipe sections used in oil fields (for example long sections of well casing or tubing) usually have a tapered, exteriorly-threaded male end called a pin member. Such pin members are threaded into couplings, collars or integral female pipe sections, their threaded ends are called a box member. These box members have an interiorly-threaded tapered thread corresponding to their respective pin members.
A dominant type of connection for these joints is the American Petroleum Institute ("API") threaded and coupled connection that achieves its assembly without torque shoulders. These tapered connections provide increasing bearing stresses to provide the seal between the pin member and box member with increasing engagement produced by rotational torque. It is well known in the petroleum industry that the performance of an API connection is highly dependent on the make-up assembly (engagement) condition of the joint, and therefore it is important to determine if the Joint is made-up properly. Assembly conditions include friction-related factors such as thread dope, plating type and thickness, surface finishes, eccentricity, ovality, impurities (dirt or rust) and external factors such as stab alignment and wind loading that occur at the well site.
Several types of methods nave been used to monitor and control make-up of oilfield tubular connections. One type of method is the "torque-only" method based upon the read-out of a load cell attached to the Joint or power make-up tongs and calibrated for torque. This method has limitations because it does not provide enough information to distinguish quality control problems such as out-of-tolerance threads, cross-threading, or galling.
A second method, "torque-turn", requires sophisticated electronics including a computer and sensors to monitor both the torque and turns which add to operational costs and delay the running time of the pipe sections. The "torque-turn" method is extremely sensitive to a reference torque which is a relatively low value, typically 10 percent of the minimum torque. This torque is sometimes determined by API torque recommendations. After this reference torque is reached, a predetermined number of turns are counted in the make-up of the tubular connection. If a false reference torque occurs to activate the turn counter because of one of the above described quality control problems or assembly conditions, an improper Joint make-up will result. An example of "torque-turn" monitoring is described in U.S. Pat. No. 3,368,396 to Van Burkleo et al. isused Feb. 13, 1968.
A third method is where the torque imposed on premium thread connections between tubular joints is monitored and plotted as a function of time rather than the number of turns. In this manner, the torque at which shoulder by metal-to-metal sealing contact is achieved during make-up of the connection can be detected. Further, torque response of the connection after shoulder may be monitored. An example of this kind of "torque-time" monitoring is described in U.S. Pat. No. 4,738,145 to Vincent et al. issued April 19, 1988.
Neither the torque-only, torque-turn nor the torque-time methods address the issue of allowing the operator to determine the amount of pin member axial engagement or positioning into the box member upon make-up of the joint. This is important in determining the amount of radial thread interference and whether the ends of the members have undesirably "butted" together, thereby restricting the bore of the pipe sections or whether there is sufficient thread engagement to withstand subsequent pressure and tensile loading.
U.S. Pat. No. 4,127,927 to Hauk et al. issued Dec. 5, 1978 discloses a fourth method that uses a combination of torque ranges and axial positioning to determine proper joint make-up In the axial relative positioning of the pipe sections, a hand tight plane is used as a reference for determining the position of a mark or marks on the pipe section(s). When in the hand tight engagement, the threads have been interengaged to a point where they are in intimate contact but without deformation, preferably reached between 25 to 50 foot pounds. Experience has shown that these relatively low reference torques result in significant variations, even on virtually identical connection specimens.
Hauk '927 discloses a complicated and expensive apparatus, preferably used at the wellsite, that gages this hand tight plane reference on each individual pin member and then marks each pin member a desired distance from the predetermined hand tight plane. The desired distance from the hand tight plane is determined empirically by making up numerous joints of each type, grade and size of pipe. Because of the variables found in the manufacturing tolerances in tubular connections each tubular joint could have a different hand tight plane reference and therefore a different position on the pipe section for the mark. As is now apparent, Hauk's method requires a time consuming analysis for the marking of each pin member prior to the initial running of the string of pipe sections.
Additionally, the Hauk method uses standardized make-up torques established by the API for each size, weight and grade of casing and tubing. (Hauk, column 1, lines 43-46; column 12, lines 45-57; column 13, lines 35-42.)
Hauk method (as disclosed in column 14, line 26 to column 15, line 16) teaches torquing a collar upon the pin member until the measured API torque reaches a preselected value. The collar end is then examined for registry with a painted-on line. This painted-on line is applied by reference to the gage (hand tight plane). The torque range is 0.75 to 1.25 times (x) the API optimum torque for the size, weight and grade of pipe. (Hauk column 14, lines 9-14.) In Hauk both torque and degree of engagement are monitored; torque by means of a torque gage and position by means of the gage-referenced mark.
Even while using these above methods for making up joints, the industry still suffers problems when forming the joints. These problems include the influent and effluent leakage because of lack of good sealing in improperly made-up joints.
There has been a long-felt need in the industry for a simplified method of determining in the field the propriety of Joint make-up visually, thereby avoiding the need for complex instrumentation such as used in the "torque-turn" or "torque-time" methods or the need to calibrate each individual pin member for proper joint make-up as in the Hauk method.
It would greatly simplify field operations in terms of time and economy if a method of determining proper joint make-up could be devised which permitted the standardizing of the marking or registering on a pipe section for a certain type, size, weight and grade of pipe sections, thereby eliminating the undesirable hand tight plane reference and the highly variable reference torque, in combination with an empirically determined torque range for a certain type, size, weight and grade of pipe sections to achieve a proper joint make-up.